Programming languages should be designed not by piling feature on top of
feature, but by removing the weaknesses and restrictions that make additional
features appear necessary. Scheme demonstrates that a very small number
of rules for forming expressions, with no restrictions on how they are
composed, suffice to form a practical and efficient programming language
that is flexible enough to support most of the major programming
paradigms in use today.

Scheme
was one of the first programming languages to incorporate first-class
procedures as in the lambda calculus, thereby proving the usefulness of
static scope rules and block structure in a dynamically typed language.
Scheme was the first major dialect of Lisp to distinguish procedures
from lambda expressions and symbols, to use a single lexical
environment for all variables, and to evaluate the operator position
of a procedure call in the same way as an operand position. By relying
entirely on procedure calls to express iteration, Scheme emphasized the
fact that tail-recursive procedure calls are essentially gotos that
pass arguments. Scheme was the first widely used programming language to
embrace first-class escape procedures, from which all previously known
sequential control structures can be synthesized. A subsequent
version of Scheme introduced the concept of exact and inexact number objects,
an extension of Common Lisp's generic arithmetic.
More recently, Scheme became the first programming language to support
hygienic macros, which permit the syntax of a block-structured language
to be extended in a consistent and reliable manner.

To help guide the standardization effort, the editors have adopted a
set of principles, presented below.
Like the Scheme language defined in Revised5 Report on the Algorithmic Language Scheme [14], the language described
in this report is intended to:

allow programmers to read each other's code, and allow
development of portable programs that can be executed in any
conforming implementation of Scheme;

derive its power from simplicity, a small number of generally
useful core syntactic forms and procedures, and no unnecessary
restrictions on how they are composed;

allow programs to define new procedures and new hygienic
syntactic forms;

support the representation of program source code as data;

make procedure calls powerful enough to express any form of
sequential control, and allow programs to perform non-local control
operations without the use of global program transformations;

allow interesting, purely functional programs to run indefinitely
without terminating or running out of memory on finite-memory
machines;

allow educators to use the language to teach programming
effectively, at various levels and with a variety of pedagogical
approaches; and

allow researchers to use the language to explore the design,
implementation, and semantics of programming languages.

In addition, this report is intended to:

allow programmers to create and distribute substantial programs
and libraries, e.g., implementations of Scheme Requests for
Implementation, that run without
modification in a variety of Scheme implementations;

support procedural, syntactic, and data abstraction more fully
by allowing programs to define hygiene-bending and hygiene-breaking
syntactic abstractions and new unique datatypes along with
procedures and hygienic macros in any scope;

allow programmers to rely on a level of automatic run-time type
and bounds checking sufficient to ensure type safety; and

allow implementations to generate efficient code, without
requiring programmers to use implementation-specific operators or
declarations.

While it was possible to write portable programs in Scheme as
described in Revised5 Report on the Algorithmic Language Scheme, and indeed portable Scheme programs were written
prior to this report, many Scheme programs were not, primarily because
of the lack of substantial standardized libraries and the
proliferation of implementation-specific language additions.

In general, Scheme should include building blocks that allow a wide
variety of libraries to be written, include commonly used user-level
features to enhance portability and readability of library and
application code, and exclude features that are less commonly used and
easily implemented in separate libraries.

The language described in this report is intended to also be backward
compatible with programs written in Scheme as described in Revised5 Report on the Algorithmic Language Scheme to
the extent possible without compromising the above principles and
future viability of the language. With respect to future viability,
the editors have operated under the assumption that many more Scheme
programs will be written in the future than exist in the present, so
the future programs are those with which we should be most concerned.

We also thank Betty Dexter for the extreme effort she put into
setting this report in TeX, and Donald Knuth for designing the program
that caused her troubles.

The Artificial Intelligence Laboratory of the
Massachusetts Institute of Technology, the Computer Science
Department of Indiana University, the Computer and Information
Sciences Department of the University of Oregon, and the NEC Research
Institute supported the preparation of this report. Support for the MIT
work was provided in part by
the Advanced Research Projects Agency of the Department of Defense under Office
of Naval Research contract N00014-80-C-0505. Support for the Indiana
University work was provided by NSF grants NCS 83-04567 and NCS
83-03325.